How many days after a tick bite should a test for Borrelia be taken? - briefly
Serologic testing for Borrelia is generally performed 2–3 weeks after a tick bite, when antibodies are likely detectable. Testing earlier may produce false‑negative results.
How many days after a tick bite should a test for Borrelia be taken? - in detail
The interval between a tick attachment and a reliable laboratory assessment for Borrelia infection depends on the diagnostic method employed and the expected kinetics of the pathogen’s markers.
Serologic assays (ELISA followed by confirmatory Western blot) detect host antibodies. IgM antibodies typically appear 2–3 weeks after exposure, while IgG seroconversion may require 4–6 weeks. Consequently, a blood sample drawn before day 14 frequently yields false‑negative results. Most clinical guidelines advise obtaining the first serology no earlier than 21 days post‑bite, with a repeat test at 4–6 weeks if the initial result is negative and symptoms persist.
Molecular techniques (PCR) target Borrelia DNA in skin biopsies, blood, or cerebrospinal fluid. PCR can identify the organism within days of the bite, especially when the erythema migrans lesion is present. However, the sensitivity of blood PCR declines rapidly after the acute phase, making it unsuitable for routine screening beyond the first week.
A practical testing schedule:
- Day 0–7: If a characteristic rash or acute neurological signs develop, obtain a skin biopsy or blood PCR immediately.
- Day 14–21: Perform the first ELISA; interpret negative results with caution, recognizing the window period.
- Day 28–42: Repeat serology if the initial test was negative and clinical suspicion remains; confirm positive ELISA with Western blot.
- Beyond 6 weeks: Re‑evaluate serology only if new or worsening manifestations occur.
Early testing without appropriate timing risks false‑negative outcomes, delaying treatment. Aligning specimen collection with the known appearance of antibodies or detectable bacterial DNA ensures the highest diagnostic yield.